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My router works fine because I can connect to QL servers from my second computer with kubuntu. I've tried firefox and qlprism and few plugin reinstallations without any results.
My system is Slackware64 13.37.

Well I don't think I can help you, then. It seems like the handshake never happens because your gamestate requests never reach the server or the server's response never reach you.
I'd join #quakelive (IRC Quakenet) if I was you to meet sponge and try to debug it with him to see if the issue is server related. Good luck.

Maybe somebody can advice me where can I find some logs or other QL messages generated during connection?
I haven't found anything interesting in the /var/log files.
error.log from ~/.quakelive/quakelive/home/baseq3 contains following informations (I can't find anything helpful):

MTU setting? Some IDS maybe (anything that could see a packet and drop it based on contents)? Or maybe even restart router, doesn't matter that it works for another computer (some stale tracked connections or something like that can still happen for one computer on the network only). Otherwise use any sniffer (like `tcpdump udp`) to see how the process of connecting differs between your two machines, that can help you find the cause.

I have tried router restarts and tcpdump sniffing without any results. But my MTU has been to low. Increasing MTU solves my problem. Thank you very much Lam for the helpful advice.

EDIT:
Maybe I should explain some details for non-advanced slackware users. DHCP daemon decreases MTU value. To avoid it one should comment line
option interface_mtu
in /etc/dhcpcd.conf so it should be:
#option interface_mtu

It should be enough because default MTU value is 1500. If one wants to change it manually, one can set it by ifconfig eth0 mtu 1500 command (ofcourse eth0 and 1500 can be replaced by appropriate values).
One can also set MTU in startup scripts. One should edit /etc/rc.d/rc.inet1.conf and add line MTU[0]=1500 in section concerning the appropriate interface (the number in brackets denotes number of the network interface).

I have tried router restarts and tcpdump sniffing without any results. But my MTU has been to low. Increasing MTU solves my problem. Thank you very much Lam for the helpful advice.

EDIT:
Maybe I should explain some details for non-advanced slackware users. DHCP daemon decreases MTU value. To avoid it one should comment line
option interface_mtu
in /etc/dhcpcd.conf so it should be:
#option interface_mtu

It should be enough because default MTU value is 1500. If one wants to change it manually, one can set it by ifconfig eth0 mtu 1500 command (ofcourse eth0 and 1500 can be replaced by appropriate values).
One can also set MTU in startup scripts. One should edit /etc/rc.d/rc.inet1.conf and add line MTU[0]=1500 in section concerning the appropriate interface (the number in brackets denotes number of the network interface).

tomo_s many thanks for the mtu info i have had exactly the same problem, ofc i have checked everything router/ports/iptables/this and that/.... LOL even diging deep with Wireshark in the network o_O this was so f**ing frustrating. if i have boot up m$-win7 i was able to play but in linux "Awaiting Gamestate..." for ever! The MTU change solved it, THX!. How you found out it is the MTU size that cause it ? I was seaching for a solution long long time and was not able the fix it.